Recently, a method using plasma has been noticed as one of these film forming methods, which is called the plasma chemical vapor deposition (CVD) method which puts the source gas or reactant gas in the plasma state to form extremely chemically active excitation molecule, atom, ions or radicals so as to promote chemical reaction, thereby growing the thin film on a substrate. This method is largely characterized in that since a chemically active ion or a neutral radical is used in the plasma state, the thin film is producible at a low temperature. Although the conventional CVD method may often be limited to a restricted process due to high-temperature formation, the plasma CVD method, which need not put the substrate at high temperature, can perform low-temperature formation, reduce thermal damage, restrict reaction between the substrate materials, and form the thin film onto a non-heat-resistant substrate. In a case where the thermodynamic reaction, even when possible, is remarkably slow, since the plasma excitation state is related thereto, the reaction may be promoted or the reaction normally considered to be thermodynamically difficult may be possible, so that novel material having various composition ratios conventionally considered impossible can be developed or a thin film of high-temperature forming material can be produced.
In the conventional plasma CVD method, a dc glow discharge method which applies dc voltage at a relatively high degree of vacuum of 0.1 to several torr between the parallel electrodes disposed within a reaction chamber to thereby perform glow discharge and a high frequency glow discharge method, which applies high-frequency power onto a coil coiled on the outer periphery of the reaction chamber formed with a quartz tube to thereby carry out non-electrode discharge, have mainly been utilized. In such methods, pressure or partial pressure of reactant gas within the reaction chamber is set low for easiness of generating plasma, whereby the thin film formation speed, somewhat different in the kinds of reaction gas, is very slow as several hundred to several thousand .ANG./min. Also, since the ion acceleration direction is orthogonal to the substrate in the glow discharge method, there is a fear that ions of reactant gas accelerated to high energy cause lattice defects in the thin film on the substrate, the sputtering causes damage thereon, or carrier gas is accelerated as ions so as to be taken in the thin film.
A light excitation plasma CVD method by laser or the like has hitherto been utilized according to remarkable progress of the laser, which is larger in photon energy due to a short wavelength of the light, so that the reactant gas is directly excited along the optical path to thereby form the neutral radicals or ions of reactant species, resulting in that it is difficult spatially control the location of film formation. Therefore, the problem is created in that a film may grow at an incident window through which the light is introduced into the reaction chamber and further that the plasma is hard to generate unless a relatively high degree of vacuum is applied as the same as the above-mentioned, resulting in that the film formation speed is also low.